1. Field of the Invention
This invention relates generally to a magnetic disk drive and more particularly to a seal structure of a spindle motor for driving magnetic disks.
2. Description of the Related Art
Recently, a magnetic disk drive as one kind of external memories is made smaller and thinner. At the same time, a smaller electrical power consumption of is required. Also, improvement of recording density of the magnetic disk is demanded in order to fulfill the requirement of a larger capacity. This results in increase in the number of magnetic disks to be loaded on the magnetic disk drive. A computer disk drive generally employs a contact start stop (CSS) system for regulating a relation between a head and a magnetic disk. According to this system, the head floats above the magnetic disk during rotation of the magnetic disk, keeping a very small space between the head and the magnetic disk, the floating force caused by an air flow generated by high-speed rotation of the magnetic disk which is balanced with the force of a spring arm.
When the magnetic disk stops rotating, the head moves on the magnetic disk and contacts the disk at a contactable area thereof. During the time the magnetic disk stops rotating, the head is kept in contact with the magnetic disk. During rotation of the disk, since the head floats a very small distance above the magnetic disk, a head crash is liable to occur due to a existence of dust, etc. As means for preventing the head crash, the magnetic disks, as well as the magnetic heads for writing and reading data to and from the magnetic disks, are disposed in a sealed chamber defined within a disk enclosure (housing).
The magnetic disks are rotated by a spindle motor assembly. The spindle motor assembly includes a spindle shaft secured to the housing, and a spindle hub rotatably mounted on the spindle shaft through a pair of bearings. By inserting the magnetic disks and annular spacers alternately into the spindle hub and tightening a clamp to the spindle hub using screws, the magnetic disks are secured to the spindle hub with a predetermined space.
A stator having a coil is fixed to the spindle shaft, and a yoke, a permanent magnet and a bushing are fixed to the spindle hub, thereby constituting a rotor. The rotor is rotatably supported on the spindle shaft through a pair of bearings. In order to prevent dispersion of oil mist from grease applied to the bearings, a pair of magnetic seals are disposed on outer sides of the bearings.
In the spindle motor assembly thus constructed, since the pair of magnetic seals are axially spacedly provided on the spindle shaft in order to prevent dispersion of oil mist, it is difficult to obtain a sufficient axial dimension for accommodating stator and therefore, an amount of magnetic flux effective for rotating the rotor is inevitably decreased. As a result, it becomes difficult to reduce the motor's consumption of power.
Several magnetic disk drives, which employ labyrinth seals instead of magnetic seals, are disclosed in Japanese Laid-Open Patent Application Nos. Sho 57-105863, Sho 62-40683 and Hei 1-277374, and U.S. Pat. Nos. 4,535,373 and 4,922,406.
However, since those conventional magnetic disk drives have a pair of labyrinth seals axially spacedly provided on a spindle shaft between a stator portion and a rotor portion of a motor, the following problems are involved. Referring to a schematic view of FIG. 1, a spindle hub 4 is rotatably mounted on a spindle shaft 2 through a pair of bearings not shown. A labyrinth seal 5 is provided between the spindle hub 4 and a base 6. Another labyrinth seal 7 is provided between the spindle hub 4 and a cover 8.
The above arrangement, in which the pair of labyrinth seals 5 and 7 are axially spacedly provided, produces an air circulation as indicated by arrows A chiefly caused by difference in pressure and temperature within a disk enclosure. Once this air circulation occurs, oil mist of the grease tends to flow into a sealed chamber to cause a head crush, etc., no matter how precisely designed the labyrinth seals employed are.